Zinc adsorption by perlite: Effects of pH, ionic strength, temperature, and pre-use as growth substrate

Zinc attracts a lot of interest in diverse disciplines of the scientific community. On the one hand, it is an essential micronutrient for plants, animals and humans; on the other hand, it is a soil pollutant. We investigated the roles of time, pH, ionic strength, and temperature in determining Zn partitioning between the solution and solid phases of suspensions of a representative plant-growth substrate (perlite). Zinc adsorption by perlite was dependent on pH, ionic strength, and temperature; it involved a combination of specific chemical affinity to adsorption sites and an electrostatic component that is related to the surface charge and is controlled by pH. Elevating temperature significantly and systematically raised the pH and enhanced Zn adsorption. A single quadratic regression was obtained between solution Zn concentration and pH in fresh perlite suspensions, which may indicate that temperature indirectly affected Zn adsorption by elevating the pH. In contrast, no single regression could be obtained for the suspensions of used perlite, which had previously served as a growth medium, and this may indicate that temperature affected both pH and Zn adsorption. The effect of pH on the apparent activation energy (Ea) for Zn adsorption was significant and each unit increment of pH induced a 4.9 kJ mol− 1 increase in Ea. Specific Zn adsorption modified the perlite charge characteristics, therefore, Zn adsorption indirectly affected the partitioning of other ions, such as P, between the solid and the aquatic phases. A significant effect of ‘enhanced P adsorption induced by Zn adsorption’ was observed.